How could anyone want Hawkeye to leave before 1) you get any character development and 2) you get to see some awesome trick arrows?

Marvel has done a lot to push Hawkeye lately. He's also being included in the new Avengers TV show, he's not going anywhere.

The roster will be;
The same + Falcon (unless he dies in the Captain America movie at the hands of Winter Soldier).

Depending on Ant-Man, they may include Pym, but I'm not sure.

I expect Thanos to defeat the Guardians of the Galaxy as well, so I don't see them joining the fight.

Where are you getting Falcon and Hawkeye appearing in the SHIELD TV show? (Note that it *is* a SHIELD TV show, and Joss Whedon has made it abundantly clear that it doesn't have a thing to do with the Avengers.) The only characters that have been confirmed for the show are a bunch of SHIELD agents who were invented specifically for TV, and who have no comic-book counterparts.